1. Field of the Invention:
This invention relates to a power converter apparatus for a DC power transmission system or a frequency conversion system.
2. Description of the Prior Art:
In the present power industry, larger power transmission has become an important subject. In large power transmission, both AC and DC power transmission systems may be found. However, in recent years, DC power transmission systems have been employed because of the following reasons:
(1) A DC power transmission system has no problem with stability so that power transmission capacity can be increased up to the limit of its current capacity.
(2) In DC power transmission, no reactive power results in minimized voltage regulation and losses caused by reactive power can never occur.
(3) The overhead lines in a DC power transmission system have by far a higher flashover voltage than those of an AC power transmission system, so that the cost required for insulation can be extremely reduced.
A DC power transmission system is provided with a rectifier and an inverter interconnected through DC power transmission lines so as to deliver and receive power. A frequency conversion system is provided with a rectifier and an inverter connected directly without DC power transmission lines so as to perform the same function. Therefore, a description will be made as to a DC power transmission system which has the above-described advantages, and those skilled in the art will appreciate the same applies to a frequency conversion system.
FIG. 1 shows a schematic configuration of a DC power transmission system with converter apparatus and the control apparatus therefor. In FIG. 1, the DC sides of converters 1A and 1B are connected to each other by way of DC power transmission lines 3 through respective DC reactors 2A and 2B. The AC sides of converters 1A and 1B are connected through converter transformers 4A and 4B, and circuit breakers 5A and 5B to respective AC power systems 6A and 6B.
The converters 1A and 1B are provided with margin angle limiter control circuits 7A and 7B, and constant current control circuits 8A and 8B, respectively. Margin angle limiter control circuits 7A and 7B limit the margin angles of converters 1A and 1B so as not to become below minimum margin angle reference values .gamma. such that converters 1A and 1B can never cause commutation failure. The minimum margin angle reference values .gamma. (to be later described) are established by margin angle reference setters 9A and 9b connected to margin angle limiter control circuits 7A and 7B, respectively.
A voltage corresponding to a current reference value which is the output of a constant power control circuit 10 and voltages corresponding to DC currents which are detected by DC current detectors 11A and 11B and in turn converted by current/voltage conversion circuits 12A and 12B are respectively fed into adders 13A and 13B. Difference outputs from adders 13A and 13B are fed into the constant current control circuits 8A and 8B. Adders 13A and 13B further receive through switches 14A and 14B the outputs of current margin setters 15A and 15B, which determine whether converters 1A and 1B are operating as rectifiers or inverters. One of converters 1A and 1B which is connected to a closed one of switches 14A and 14B is operated as an inverter, and the other converter with switch 14A or 14B opened is operated as a rectifier.
Assume now that the switch 14A is opened while the switch 14B is closed. In this case, a control advanced angle preference circuit 17A outputs an input from constant current control circuit 8A. The output of control advanced angle preference circuit 17A is fed into a phase control circuit 18A so as to be converted into a pulse signal that determines trigger timings of converter 1A, which is fed through a pulse amplifier 19A into converter 1A as a gate pulse signal. In the case where converter 1A is operated as an inverter, the control advance angle preference circuit 17A outputs the smallest value among the outputs of constant current control circuit 8A, margin angle limiter control circuit 7A and a constant reactive power control circuit 16.
Since the switch 14B is closed, converter 1B is operated as an inverter. In this case, a control advanced angle preference circuit 17B outputs the smallest value among the outputs of margin angle limiter control circuit 7B, constant current control circuit 8B and constant reactive power control circuit 16. The output of constant reactive power control circuit 16 becomes necessary when converter 1B is operated so as to perform reactive power control of the AC power system. In this case control advanced angle preference circuit 17B outputs an input from constant reactive power control circuit 16, and circuit 17B produces, on the basis of the received signal, a phase signal and feeds the same to a phase control circuit 18B. Phase control circuit 18B converts the received phase signal into a pulse signal that determines trigger timings of converter 1B. The pulse signal is fed through a pulse amplifier 19B into converter 1B as a gate pulse signal.
The above-described configuration of control circuits for converters is known as a prior art, and it is also a well-known fact that the operation characteristic curves of such a DC linkage system is as shown in FIG. 2, wherein the abscissa designates DC current Id, and the ordinate DC voltage Ed.
In FIG. 2, a line a-b-c represents an operation characteristic curve of converter 1A when it operates as a rectifier (because switch 14A is assumed to be opened, converter 1A is a rectifier). The line a-b is a portion of regulation which is determined by a commutation impedance that includes a converter transformer 4A. The line b-c is a portion of constant current characteristic determined by the operation of the constant current control circuit 8A. A line d-e-f represents an operation characteristic curve of converter 1B when it operates as an inverter (because switch 14B is assumed to be closed, converter 1B is an inverter). The line d-e is a portion of constant current characteristics determined by the operation of constant current control circuit 8B. The line e-f is a portion of constant margin angle characteristics of converter 1B determined by the operation of margin angle limiter control circuit 7B. Here, the difference between the points c and d on the abscissa of the operation characteristic curve in FIG. 2, which represents the difference of DC current Id corresponds to the current margin.
The converters in the DC power transmission system are operated at the point A (in FIG. 2) which is the intersection of the operation characteristic curves of converters 1A and 1B. Converters 1A and 1B of the DC power transmission system is generally provided with constant power control circuit 10 in order to control transmission power to be shared between AC power systems 6A and 6B. The power reference value established by an active power reference setter 20 and a detected power value produced from a power detector 21 that detects transmission power are fed, with polarities opposite to each other, into adder 22. The difference output of adder 22 is amplified within constant power control circuit 10 so as to become the above-described current reference value. This configuration allows the transmission power to be controlled in accordance with the power reference value.
As can seen from the operation characteristic curves shown in FIG. 2, the converter operated as an inverter determines the DC voltage, while the converter operated as a rectifier determines the DC current so as to control power transmission.
However, converters are considered, when operating as either rectifiers or inverters, to be a lag load when observed from the respective AC power systems, and the power factors thereof are wellknown to be substantially proportional to the cosine of a control delay angle or that of a control advance angle. Therefore, a reactive power control circuit is provided to control the reactive power. A reactive power reference value established by a reactive power reference setter 23 is fed, with a polarity opposite to the reactive power detected value produced from a reactive power detector 24, into an adder 25, and the difference output thereof is amplified within a constant reactive power control circuit 16, and in turn, fed into control advanced angle preference circuits 17A and 17B.
Although not shown in FIG. 1, it is naturally practiced that when the reactive power of AC power system 6A is controlled, the reactive power thereof is detected by reactive power detector 24, and when the reactive power of AC power system 6B is controlled, the reactive power thereof is detected. Even when the reactive power of AC power system 6A is controlled under such a condition that converter 1A is operated as a rectifier, should the margin angle of converter 1B be controlled in accordance with the output of constant reactive power control circuit 16, the control angle of converter 1A varies so as to follow the margin angle of converter 1B, so that the reactive power of AC power system 6A can be naturally controlled.
Assuming that when both converters 1A and 1B are operated at point A in FIG. 2 and the control delay angles thereof are increased by the operation of constant reactive power control circuit 16 in order to increase lagging reactive power which is consumed within converter 1B, the DC voltage is lowered, so that the line d-e-f of the operation characteristic curve shifts to the line d'-e'-f'. Also, constant power control circuit 10 increases, in order to cause transmission power to follow the power reference value, the DC current increases so as to compensate for the lowered DC voltage, and as a result of this, the line a-b-c of the operation characteristic curve of the converter 1A shifts to the line a-b'-c', consequently operation point A of both converters shifts to point A'. (The transmission power can be considered as the multiplification of the DC voltage and the DC current, so that a curve of constant power becomes a hyperbola on which the operation points of both converters are invariably present as shown in FIG. 2).
As is known, the above-described DC power transmission system is provided with shunt reactors and shunt capacitors as components of phase-modifying equipment. Shunt reactors 26A and 26B through circuit breakers 25A and 25B, and shunt capacitors 28A and 28B through circuit breakers 27A and 27B are respectively connected to respective AC power systems 6A and 6B. It is also generally known that the number of units of phase-modifying equipment and the capacity thereof depend upon the specifications of system operations.
Prior-art control methods have provided for phase-modifying equipment such that on the basis of the power reference value established by the power reference setter 20 and the detected value derived from the active power detector 21 in FIG. 1, ON-OFF controls of phase-modifying equipment are uniformly performed. For example, when the power reference value set by power reference setter 20 is less than 30%, only shunt reactor 26B is closed. When it is in the range of 30% to 70%, both shunt reactor 26B and shunt capacitor 28B are interrupted. When it is more than 70%, only capacitor 28B is closed.
The above-described prior-art control method is relatively simple and has no problem in the case of a constant DC voltage control or a constant margin angle control wherein a reactive power control or an AC power system voltage control is not performed. However, in the case of the above-described system wherein reactive power is controlled, the method has various disadvantages which will be described hereinafter.
FIG. 3 is a diagram illustrating the capability of controlling reactive power by the converters in FIG. 1. In FIG. 3, the abscissa designates transmission power P, and the ordinate reactive power Q. For the sake of simplicity, shunt reactor 26B and shunt capacitor 28B are assumed to be equal in capacity.
Although not shown in FIG. 1, converters generate harmonics, so that there are usually provided AC filters to absorb the harmonics. The AC filters can be considered as leading reactive power sources, so that in FIG. 3, the capacities corresponding to the AC filters are assumed to be certain appropriate values.
First, in FIG. 3, the hatched region surrounded by the points A, B, C, D and E represents the case where only shunt capacitor 28 is closed. The curve A-B represents the relationship of P-Q in the operation at a minimum margin angle (hereinafter simply referred to as .gamma. min). The represents the P-Q curve in the operation at a maximum margin angle corresponding to the capability of a continuous operation with a 100% DC current of the converter. The straight lines A-E and B-C respectively represent the minimum limit and the maximum limit of the transmission power. The curve C-D represents the limit of constant operation with a 100% DC current.
When shunt capacitor 28B is interrupted, the hatched region moves downward substantially in parallel. Here, the reactive power controllable region under the condition in which both shunt reactor 26B and shunt capacitor 28B are interrupted is represented by the region surrounded by the points M, C, N, Q and R. In addition, when shunt reactor 26B is closed, the thus moved region further moves downward substantially in parallel. Namely, the reactive power controllable region under the condition in which shunt reactor 26B has been closed is represented by the region surrounded by the points G, H, I, J and K.
Here, assumption is made such that shunt capacitor 28B and shunt reactor 26B are not closed, that the system is operated with a power reference value (output value) which is less than 70%, and that reactive power is operated at a point corresponding to the point C' in FIG. 3. When the system is required to operate at 80% of a power reference value (output value), the operation cannot be performed within the region surrounded by the points M, C, N, Q and R, so that shunt capacitor 28B inevitably becomes closed at the 70% power line connecting points B' and D'. When shunt capacitor 28B is closed, leading reactive power is supplied to the system, so that the operation region thereof moves upward substantially parallel. In this case, the operation region becomes the region surrounded by the points B', B, C, D and D'. Next, in the case where the system load is reduced and the system is operated at 60% of a power reference value (output value), shunt capacitor 28B is interrupted at the 70% line connecting points B' and D', consequently the operation region moves again downward.
In this case, the converters can no longer be operated within the region surrounded by the points A, B', C', F and E since in this region, the transmission power is less than 70% yet operation in this region defined by points A, B, C, D and E is controlled to occur only when the system is operated at greater than 70% of the power reference value. This causes such a disadvantage that the reactive power controllable region of the converters is inevitably restricted and decreased by the control of phase-modifying equipment to a considerable extent.